1. Field of the Invention
The present invention relates to a collision detection device which is designed to detect collision of a moving body based on the detection of acceleration above a certain level acting on the device. This collision detection device is used, for example, to trigger the activation timing device of the air bag system or seat belt tensioner which protects passengers in the vehicle in the event of collision.
2. Related Art
Among conventional mechanical-type collision detection devices used for activating air bags, Japanese patent publication JP-A 8-264088 discloses a device, which includes a pendulum (termed as "weight" hereinafter) pivoted at a position eccentric from its barycenter so that it swings in response to an acceleration above a certain level acting on it in a certain direction. A cam rotor turns together with the weight and operates to close associated electrical contacts, thereby producing a collision detection signal.
This collision detection device include a contact spring which exerts a force on the weight against the swing motion caused by the impact of collision thereby defining a threshold of detection and restricting the weight from swinging when the vehicle does not actually collide, thereby preventing the erroneous detection. A stopper is provided on the path of swing motion of the weight so that the swing motion at the impact of collision is limited thereby.
The above-mentioned mechanical-type collision detection device is mainly used as a redundant safety sensor to back up an active electronic acceleration sensor, and in this case it is designed to have a weight of a large eccentric mass moment and a moment of inertia that is small enough to swing at the incident of collision.
However, the conventional collision detection device of the above-mentioned structure may not meet the demand satisfactorily in some cases. For example, if the collision detection device is installed in the vehicle crash zone or is used to detect the incident of side collision, the weight, with its large eccentric mass moment and small moment of inertia, will have a large swing velocity and thus will have a large bouncing force by when it impacts the stopper with short duration, large-amplitude acceleration. The resulting reciprocating motion of the weight induces the chattering of electric contacts and produces an unstable pulsative collision detection signal.
Particularly, in an air bag system where a collision detection signal is subjected to logical-product gating with other sensor signals in triggering an inflator, the unstable collision detection signal can possibly fail to inflate the air bag.
In order to restrict the reciprocating motion of the weight when it undergoes the impact of short duration, large-amplitude acceleration, it must be designed to have a large moment of inertia which depends on its shape. The amplitude and duration of acceleration caused by the impact of collision varies depending on the type of vehicle, device installation location, and direction of collision. Therefore, in order to provide a proper moment of inertia for the weight to meet individual functional conditions, it needs to be designed on a trial-and-error basis, resulting in a high manufacturing cost.